1. Field of the Invention
The present invention relates to novel aqueous microhydrogel dispersions, to processes for producing the same, and to processes for producing microhydrogels. More particularly, the invention relates to aqueous microhydrogel dispersions composed of extremely fine hydrogel particles which have not been even thought of heretofore, dispersed stably in an aqueous medium, to processes for producing the same, and to processes for producing microhydrogels.
2. Description of the Prior Art
In recent years, substantially water-insoluble and water-swellable substances (hydrogels) obtained by giving a cross-linked structure to water-soluble or hydrophilic polymers, are receiving attention in that they have characteristic properties such as affinity to water, water retension, instantaneous absorbing power of a large amount of water, moisture absorbing power and affinity to the tissues of the human body, and the utilization of said substances for various purposes is now actively studied.
Among others, the attempt to eliminate the generally observed insufficiency in the functions, such as hygroscopicity, water absorption, flame retardancy, etc., of synthetic high polymers which are starting materials for synthetic fiber products and various plastic molded articles, by mixing such hydrogels, is one of the important problems which are expected to attain success. However, it is acknowledged that, when a hydrogel is mixed with a synthetic high polymer material, the values of the dynamic characteristics, such as strength, modulus of elasticity, etc. which are indispensable as fundamental properties of the synthetic high polymer materials are lowered. To solve such problems, it has been strongly demanded to find a concrete industrial means to render the hydrogel fine in diameter.
Among other applications of the hydrogel in addition to the above-mentioned, there may be recited: body fluids absorbing materials such as diapers, sanitary napkins, hemostatic bandages, etc.; applications utilizing the ability of the hydrogel to absorb a large amount of water instantaneously, namely solidifying agents for water-containing muddy substances such as excess sludge generated when treating activated sludge of industrial waste water, human or animal excrement, etc.; applications utilizing the moisture absorbing ability of the hydrogel, such as drying agents for various articles, moisture detecting elements, etc. In such fields of uses, it has been also strongly demanded to establish an industrial means to make the hydogel fine in diameter for the purpose of increasing the water or moisture absorbing rate.
To make the hydrogel fine in diameter, several methods have been proposed, for example, a method wherein dry hydrogel granules are pulverized by means of a ball-mill, etc. and another method wherein hydrogel is dispersed in water to bring it to a swollen state and then it is pulverized by means of a shearing equipment such as a mixer, followed by drying. However, by any of these known methods, the resulting hydrogel particle diameter is usually several hundred microns and even the finest is several ten microns, so that it has been practically impossible to form a hydrogel having an extremely fine particle diameter.
Under such circumstances, the present Applicant proposed a technical means in the previously filed Japanese Patent Application No. 116,322/1976 and No. 143,772/1976, in which an aqueous solution of a hydrophilic polymer containing cross-linkable groups is mixed with a water-miscible organic solvent to precipitate said hydrophilic polymer as fine particles, and then the solvent is removed before or after and/or at the same time with said formation of cross-linked structure, whereby a hydrogel having a particle diameter not larger than 10.mu. is formed. But these previously filed inventions comprise the step of preparing an aqueous solution of a hydrophilic polymer, the step of precipitating the hydrophilc polymer by mixing a water-miscible organic solvent, the step of forming a cross-linked structure, and/or the step of solvent removal. Because of such complicated process steps, these inventions are not industrially desirable methods. Moreover, it is a problem that these methods require a great amount of water and a water-miscible solvent. Also, the diameter of the hydrogel particles formed by these methods was in the order of 1.mu. at the smallest.